feat: standalone CMake build against a released WasmEdge SDK

[0yi0]

Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Depends on the source import (see Relationships).

In the core repo, plugins build via in-tree CMake infrastructure (wasmedge_add_library, wasmedge_add_executable, WASMEDGE_PLUGIN_* options, in-tree include paths). Make them build standalone against an installed WasmEdge release — currently 0.17.0 — so plugin maintainers never track unreleased master.

• Top-level CMakeLists.txt with one WASMEDGE_PLUGIN_<NAME> option per plugin, all OFF by default (build only what you ask for — keeps the CI matrix per-plugin).
• cmake/ helpers to locate the WasmEdge SDK: honor WASMEDGE_INCLUDE_DIR / WASMEDGE_LIB_DIR hints, with an optional fallback that fetches the official release tarball for the pinned version.
• Port the helper functions the plugin CMakeLists rely on (wasmedge_add_library, wasmedge_add_executable, wasmedge_setup_target, …) into cmake/Helper.cmake with the in-tree assumptions removed.
• Port the test scaffolding (gtest setup from test/plugins/CMakeLists.txt) so plugin test suites can build.
• When porting test/plugins/CMakeLists.txt, drop or disable its unconditional add_subdirectory(wasi_logging): the test references target wasmedgePluginWasiLogging, which was not imported — upstream builds wasi_logging as a temporary built-in from lib/plugin/wasi_logging + include/plugin/wasi_logging, outside the import scope of feat: import plugin sources and tests from WasmEdge core with git history #23. Keep it disabled until that target exists here (or the matching sources are imported). Found in review: feat: import plugin sources, tests, and CI utils from WasmEdge core #44 (comment)
• Pin the minimum WasmEdge version in one place and document the upgrade policy in README: plugins sync to new WasmEdge APIs only when WasmEdge cuts a release.
• Acceptance: cmake -GNinja -Bbuild -DWASMEDGE_PLUGIN_ZLIB=ON && cmake --build build configures and compiles the zlib plugin against the installed 0.17.0 SDK on Linux. (Full pilot — tests + CI — is the follow-up pilot issue.)

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